The present invention relates to a fuel injection control method intended for a direct-injection internal-combustion engine.
More particularly, it relates to a fuel injection method allowing to obtain homogeneous mixing of the fuel injected with air or with a mixture of recirculated exhaust gas and air.
The development of internal-combustion engines has to meet requirements relative to emissions reduction, fuel consumption, torque and specific power increase, combustion noise reduction, while remaining compatible with endurance resistance criteria.
It is therefore possible to change certain parameters of the injection system (such as the pressure, the control laws, etc.) or of the fuel jets, such as the nappe angle formed by these jets at the injector nozzle.
However, improvement of one of these parameters leads to a degradation of another parameter.
Thus, this is for example the case with the injection of liquid fuel in the combustion chamber where this fuel is injected with a nappe angle determined by the type of injector used.
A modification of this angle can lead to a projection of fuel on the cylinder wall, which causes a degradation of the behaviour of the lubricant present on this wall and the formation of soots.
This has the drawback of leading to an emissions increase and/or to a reduction of the full load performances and/or to a noise increase.
As described in patent applications EP-849,448 or EP-589,178, the fuel is fed into the combustion chamber by an injector arranged in line with each cylinder, with a very wide nappe angle of the order of 140xc2x0 to 160xc2x0.
By means of this injector, the fuel jets are discharged and vaporize without ever touching the cylinder wall and therefore without affecting the lubrication thereof
To obtain this effect, the piston must of course be rather close to its top dead center (PMH), so that only a limited amount of latitude is available for the selection of the fuel injection times.
This drawback is by no means insignificant since it is also known that early and/or late fuel injections afford many advantages.
For example, a pilot injection before the top dead center and the main injection allows the combustion noise to be reduced.
Furthermore, as it is well-known, the regeneration of particle filters, initiated by an increase of the temperature at the exhaust, requires a fuel injection shortly before opening of the exhaust valves.
A fuel injection upon expansion or during the exhaust phase can also be useful to obtain favourable conditions at the exhaust for regeneration of the NOx, traps.
Furthermore, in order to obtain a homogeneity of the fuel mixture, it may also be desirable to inject fuel early.
Since the injectors used generally have a nappe angle ranging between 140xc2x0 and 160xc2x0, the injection control range is reduced so as to limit problems of lubricant dilution by fuel.
The present invention is aimed to overcome the aforementioned drawbacks by means of a fuel injection method allowing to use very varied injection conditions.
To this effect, a fuel injection control method intended for a direct-injection internal-combustion engine comprising at least an injector of fuel jet nappe angle a1, a cylinder, a cylinder head, a piston sliding in this cylinder and connected to a crankshaft, and a combustion chamber delimited by the wall of the cylinder, the cylinder head and the upper face of the piston, is characterised in that it consists in:
determining a lower limit position of the piston below which the cylinder wall is likely to be wetted by the fuel jets from the injector,
determining the effective position of this piston,
locating this effective position of the piston in relation to the lower limit position,
adjusting the fuel injection parameters according to the effective position of the piston in relation to the lower limit position.
Advantageously, the method can consist in adjusting the injection parameters so as to limit the penetration of the fuel jets in the combustion chamber for any position of the piston between its lower limit position and its furthest position from the cylinder head.
Preferably, it can consist in injecting a determined amount of fuel under very high pressure, preferably above 1000 bars.
This method can consist in injecting a determined amount of fuel for a very short length of time.
It can consist in repeating the injection of a determined amount of fuel until the total amount of fuel to be injected is obtained.
The determined amount of fuel may not exceed 25% of the total amount of fuel to be injected.
Advantageously, it can consist in determining the lower limit position of the piston by means of the relation Dxc3x97tan(a1/2), where D is the radial distance between the point of origin of the fuel jets and the cylinder wall.
The method can consist in using injection parameters to inject the fuel conventionally for any position of the piston between its lower limit position and its closest position to the cylinder head.